The radio frequency (RF) spectrum is a limited commodity. Only a small portion of the spectrum can be assigned to each communications industry. The assigned spectrum, therefore, must be used efficiently in order to allow as many frequency users as possible to have access to the spectrum. Multiple access modulation techniques are some of the most efficient techniques for utilizing the RF spectrum. Examples of such modulation techniques include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA).
Wireless service providers also seek other ways of using the available spectrum as efficiently as possible. One important technique for maximizing spectral efficiency is to minimize overhead message traffic. If the number of overhead messages transmitted is reduced, fewer overhead channels are required to carry overhead messages. This frees up spectrum for user traffic. Also, reducing the number of overhead messages reduces the processing load in both the mobile stations and the base stations of the wireless network.
As is well known, when a wireless mobile station moves from a cell that is served by a source base station to a cell that is served by a target base station it becomes necessary to transfer or handoff the wireless mobile station from the source base station to the target base station. In most wireless networks approximately forty percent (40%) to fifty percent (50%) of all active calls experience some type of handoff. These handoffs involve adding cells or dropping cells to an active call, or handing the call over to another cell under the control of another base station. In either case a decision must be made prior to the handoff identifying which base stations are to be involved in the handoff. If the base station selection algorithm that is employed is not robust, unnecessary air signaling and call quality degradation may result. Improvements to base station selection algorithms for handoffs can result in a lower call drop rate and better overall network performance.
In a wireless network each cell will have an assigned set of frequencies on which the cell operates. A cell is referred to as a “border cell” if it is located next to a neighbor cell that does not operate on the same frequencies. For example, a wireless mobile station may leave a first area that is served by a first wireless provider that uses a first set of frequencies and enter a second area that is served by a second wireless provider that uses a second set of frequencies. When the wireless mobile station leaves a border cell of the first area and enters a first cell of the second area, an interfrequency hard handoff must be performed to transfer control of the wireless mobile station from the border cell of the first area to the first cell of the second area.
A number of prior art techniques have been used to perform an interfrequency hard handoff. One such technique is referred to as the “pilot beacon” method. In the pilot beacon method, the target cell (i.e., the first cell of the second area) transmits a pilot signal on the same frequency used by the mobile station in the border cell to assist the wireless mobile station in determining the pilot strength of the target cell. The wireless mobile station determines the pilot strength of the target cell and triggers the interfrequency hard handoff based on the pilot strength information.
There are a number of disadvantages with the pilot beacon method. Additional expense is required to provide a pilot beacon. Each pilot beacon requires separate radio frequency (RF) upconversion circuits and amplifiers. The pilot signal normally is set to fifteen percent (15%) to twenty percent (20%) of the total power target cell. Providing either continuous or discontinuous pilot signals may result in a greater amount of interference. This results in a reduction in the capacity of the wireless network. The modules that are required to support the pilot beacons are very expensive and can cost as much as several tens of thousands of dollars.
Another prior art technique for performing an interfrequency hard handoff is referred to as the “distance based” method. In the distance-based method the base station in the border cell determines the distance of the wireless mobile station from the base station in the border cell. The base station makes the distance determination from the signal strength reported by the wireless mobile station on the Pilot Strength Measurement Messages (PSMM) that the wireless mobile station sends to the base station. If the signal from the wireless mobile station is too weak because the wireless mobile station is too far from the base station, then the base station will initiate an interfrequency hard handoff to the target base station. The target base station is closer to the wireless mobile station and has greater signal strength.
One of the primary disadvantages of the distance-based method is that the distance is determined solely on the basis of the power of the received signal of the wireless mobile station. The correlation of the distance with the power of the received signal is reliable only in environments in which the power of the received signal at points distant from the base transceiver station is known. Those skilled in the art will recognize that the power of the received signal at points distant from the transmitting antenna varies inversely with the square of the distance in the far field region. Furthermore, those skilled in the art will recognize that the propagation environment between the base transceiver station and mobile stations at different locations, but with the same range, may be vastly different. Measurements have shown that the received signal power at a particular location is random and normally distributed log-normally about a mean signal power value. Generally speaking, the large statistical distribution of the signal power with distance results in an unreliable prediction of the distance based on the power of the received signal in environments with multipath effects, fading effects, shadowing effects, additive combining, and doppler effects. These types of phenomena can produce misleading pilot strength signal readings at the wireless mobile station and cause the distance-based method to lead to erroneous handoff decisions.
Another prior art technique for performing an interfrequency hard handoff is referred to as the “candidate frequency search” method. In the candidate frequency search method information from the candidate frequency search set is used. When the wireless mobile station that is located in the border cell is operating in the traffic state, the base station of the border cell instructs the wireless mobile station to measure the pilot strengths in the given candidate frequency search set. The wireless mobile station periodically measures the signal strength of the pilots in each candidate frequency search set and reports the results of the measurements to the base station using the “Candidate Frequency Search Report Message.” Depending upon the pilot strength information provided by the wireless mobile station to the base station, the base station initiates and performs an interfrequency hard handoff of the wireless mobile station to the target base station.
A serious disadvantage of the candidate frequency search method is that it requires a substantial amount of additional signaling traffic on the network and disrupts voice traffic during the transition. In addition, when the wireless mobile station is measuring the pilot strength in the different frequencies, the wireless mobile station has to sacrifice the traffic on its present frequency. This results in degraded voice and data service.
There is therefore a need in the art for an improved apparatus and method for performing an interfrequency handoff to transfer control of a wireless mobile station in a wireless network from a base station of a border cell in a first area to a base station of a neighboring cell in a second area. There is also a need in the art for an improved apparatus and method to provide an improved handoff decision control program that is capable of performing an interfrequency handoff of a wireless mobile station in a wireless network from a base station of a border cell in a first area to a base station of a neighboring cell in a second area.